Bees in the cold part 3
Guest post written by Dean Haley
In this blog post I dig deeper into the properties of wooden boxes and foam insulation. We build an insulated cover and I test this with bees in Applethorpe QLD with temperatures as low as -2.4C
In bees in the cold (part 1) I showed how much a standard hive cooled down in the fridge and how much better it performed with insulation. I decided to research styrofoam covers.
I started off by asking questions on online stingless bee forums. I am grateful for the responses received. Some beekeepers use covers and others don’t. The most common are styrofoam broccoli boxes, often cut down to size, glued with liquid nails and painted with acrylic to prevent UV damage. Most people do not cover the base, some incorporate an air gap, and everyone leaves the cover on year round. There is general agreement that a cover can trap heat and overheat in summer so airflow is recommended.
What strikes me as odd about the consensus information is that we have a breezy cover good for summer, but suddenly not breezy and good for winter too.
MY ALL-WEATHER FOAM COVER
As always Nick is my craftsman. We built this cover out of 50mm thick extruded polystyrene with an advertised R factor of 1.7
It was cut with Nicks normal wood working tools, glued with no nails and painted with acrylic. I lined parts of the cover with the blue backed reflective foil they use when building houses. In winter the sun shines on the dark blue painted surface and gently warms. This is a great place for my bees to sun bake and absorb heat. The coloured front actually pumps heat into the styrofoam cover. The foam and foil insulation reduce night time heat losses to less than 3 Watts (see my calculations later).
In summer, the front and rear panels are removed for cooling air flow. The long eave at the front provides summer shade (and winter sun). The front of the box must face North.
I used the same OATH hive that I had put in the fridge in bees in the cold part 1. I took them to Applethorpe for a week in my new cover where the official BOM temperatures ranged From -2.4 to 17.2C. As before the temperature control in carbonaria is outstanding. The red brood temperature controlled (once again) between 28 and 30 degrees. The styrofoam cover performed perfectly, conserving heat with the honeypots (blue line) never falling below 15C. There’s not much more I can say except I am excited to share these results and help my fellow beekeepers.
The following information is in depth data on insulation. This is what I used to come to an appreciation of what my bees need. It’s a bit information dense, sorry!
INSULATION & HOUSES
Have you ever looked up house insulation? If you have you will have noticed the huge amount of information out there. Some of the important concepts are;
Shade in summer
Winter sun
Thermal mass
Insulation, represented as the R value
Thermal conductivity, the k value
Air gaps
Reflective foil
Conduction, convection & radiative heat loss.
To make matters worse there are two systems out there. The values in the USA imperial system are about 5.7 times greater than the metric system in R values and no one bothers to state which system they are.
HOW DOES THE R VALUE WORK?
In the metric system, the R value relates to heat transfer across a wall and is expressed as watts per square meter.
According to wikipedia, if I have 30C on one side of a wall and 20C on the other side (a difference of 10C) and the wall has an R value of 2.0 then the watts of heat energy transmitted on the cold side of the wall is 10÷2 = 5W/m2
Simple, right?
Technically there is a little more to it. Heat flows upwards through an insulator easier than it flows downwards. Airflow over a surface upsets things and so on, but you get the idea.
HOW MUCH HEAT IS MY BOX LOSING?
Well, that depends on the surface area of the box, density of the wood and thickness of the wood.
Let’s take a standard OATH box 28cm x 20cm x 25cm high. It’s made of 25mm thick hoop pine with commonly quoted R values of 0.25 which will do for this calculation.
If we have 20C inside the box and 10C outside (10C difference) & a box surface area of 0.352m2 then heat loss is (10÷0.25) × 0.352 = 14.08 watts.
As I showed in bees in the cold part 1 (using the standard OATH) my bees just don’t generate this much heat.
CALCULATIONS
Use these calculations to see how much heat energy your box is losing. You need to know your timber density (G), timber thickness (L) and box surface area. That’s it. Easy.
Warning, warning!. These calculations I must stress are based on perfect conditions with still air and no sunshine. Hardly normal but that is how these calculations work, throughout the world and the building industries. I also assume wood has 12% moisture to simplify calculations.
In Physical Properties and Moisture Relations of Wood (Simpson.W, TenWolde.A) a calculation for thermal conductivity (k) is given (adapted by me):
k = G × 0.242868 + 0.01864
where G = specific gravity of your timber. A few example timber specific gravity follows;
Western Red Cedar = 0.33
Hoop pine = 0.56
White beech = 0.515
Spotted gum = 1.010
Thermal conductivity (k)
k = W/m/K
Where W= watts, m = metres, K = degrees kelvin.
U = k/L where L = timber thickness in metres.
U = 1/R
therefore R = 1/U
Therefore R = 1÷(k÷L)
Now you have your R value, you can calculate your box surface area (in square metres) and calculate how much heat energy your box loses at a given temperature difference.
Example:
Worked Example.
I have a 40mm thick timber (0.04m) western red cedar box (G = 0.33) with 0.352m2 surface area.
k = 0.33 × 0.242868 + 0.01864
= 0.0988
R = 1 ÷ (0.0988 ÷ 0.04)
= 0.4049
For a 10C temperature differential;
(10÷0.4049) × 0.352m2 = 8.69W heat loss.
WHAT ABOUT FOAM INSULATION?
My foam cover is sealed on all six sides. It has a 30mm air gap all around the box so the final foam box is quite large. It has a surface area of 0.8768m2 and its losing heat from this increased surface area.
The trick now is to add the two insulating R values together. 0.25 (wood) + 1.7 (foam) = 1.95
(10C ÷ 1.95) × 0.8768m2 = 4.496 watts
This is an improvement!
HOW ABOUT FOIL & AIR GAPS?
Builders foil reflects about 97% of radiant heat. In R values this adds about R = 1.1799 to your insulation but beware:
Air gaps are almost useless without foil, and foil has an R value of zero without an air gap.
Foil/ air gaps work for gaps 20mm to 50mm. Gaps larger than 50mm will lead to air circulation and convective heat loss. Adding foil to my cover design reduces heat loss to less than 3 watts for a 10C temperature differential.
THERMAL MASS BEE BOXES
Wood is an amazing building material. As well as being an insulator it has value as thermal mass. The more thermal mass you have the more it resists temperature change, both heating and cooling. Robert Luttrell Uses thermal mass in his designs.
I found some interesting information from the US relating to thermal mass and insulation qualities of log cabins. Following the standard calculations (such as I provided earlier) would predict these cabins were terribly cold where in fact they were perfectly comfortable. The building codes of the US were changed to acknowledge the unique properties of log walled cabins. They found that a cabin wall with insulating R values of 1.58 was equivalent or better than standard insulation of R 2.29
Thermal mass works in both summer and winter. It works best in mild and temperate climates. It requires a reasonable timber thickness to work.
If adding insulation to thermal mass, always add it to the outside or you will lose your thermal mass advantages.
SURFACE AREA TO VOLUME RATIO
In Artic environments animals tend to be round and cuddly, while in desserts they are lanky and lean. The cuddly animal has less surface area as a ratio of body weight so radiant heat loss is reduced. The most heat conserving bee box in a colder area would be a cube.
CONDUCTIVE HEAT LOSS
Try walking on cold tiles in winter. It’s a bit chilly isn’t it? This is conductive heat loss and can happen with your bee box too. Placing styrofoam or carpet under your box in winter is an instant win.
HEATERS
Sorry! I haven’t tried heaters. I mean I know there are people using heaters, thermostats and so on. I just haven’t gotten that far yet. If you are using heaters successfully, please send us a comment and share your knowledge with others.
IN CONCLUSION
This concludes my series of posts on bees in the cold. Thanks to Nick from Australiannativebee for building my prototype box. Thanks to Daniel Klaer at Gondwana bees for reviewing my calculations.
This final post was also aimed at my fellow beekeepers. We have established that strong carbonaria hives try to regulate its brood temperature and needs decent insulation to do this in cooler climates. The calculations in this post provides the tools for beekeepers to design better insulated boxes. Happy beekeeping.
Some useful links:
3–1 Chapter 3 Physical Properties and Moisture Relations of Wood
Great article Dean, shold have people thinking. Bob
Yet another great practical experiment, showing real results from the field.Thanks for the great research, amazing to see how well the colony could perform in a difficult temperature range when a strong colony is used. David
Great post
I was using timber oath hives, however had issues with timber deteoriating providing crevices for ceriana ornata to lay eggs. I use clay pots now, painted white with a poly capped inspection port. I’m in CQ, so cold’s not really an issue, but heat is, so I’m considering placing existing pots inside larger clay pots and filling the void between with foam filler. Another option I’m considering is placing pot hives underground.
Construction of timber hives is a tricky one. Stable timbers that do not warp with no finish applied are what I aim for but they can be expensive about $100 or more per box for materials. I would love to hear what temperature data you come up with if you try either of those ideas. Please keep me posted.